The bi-functional chelator NOTA-(p-Bn)-NCS permits radio-labelling heat sensitive proteins with Ga-68 (t1/2=68 min) for positron emission tomography. Complexation at room temperature (RT) completes within minutes and in vivo transmetalation is negligible. Here, influences of trace metal cations, on radiochemical yield (RCY) and specific radioactivity (SRA) are assessed.
Conjugation of NOTA-(p-Bn)-NCS to AnnexinA1 was performed at varying stoichiometries and conjugates purified by size exclusion high-performance liquid chromatography. Available complexation sites per protein were identified by colorimetric assay and titration with carrier added Ga-67. The complexation of Ga-68 by NOTA-AnnexinA1 at RT and 37°C was systematically studied with and without addition of competing trace
metal cations. Ga-67 was used for confirmation of observed trends. Integrity
of the radio-conjugate was assessed by addition of up to 104 fold excess of
metal cations or apo-transferin and exposure to human serum at 37°C. Gallium-68 gave RCY of N99% within minutes at RT whereas, Ga-67
yielded max 85% dropping as the stock decayed. Presence of Fe(III) showed
significant influence on RCY. Once formed, the radio-conjugate showed negligible loss of radioactivity under even the most extreme conditions investigated. SRA and RCY of the radio-conjugate depend significantly on
absence of particularly Fe(III) during complexation.